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LOP Misfires


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For the last several months I have been trying to troubleshoot an interesting issue I have with my IO-360-A3B6D in my J model. To this day, I still can't get the thing to run LOP smoothly. I've been working with Paul K. with Savvy who's been a great help! Things I've tried:

1. GAMI spread is down to .2 GPH

2. Fine wire plugs. Gap is all on the lower end of permissible range. 

The most interesting thing of all of this when looking at my G3x engine data is that multiple cylinders start to misfire around 30-40 LOP. Usually what will happen is after I get to peak on all cylinders, egt's will continue to fall. However, once they reach 30-40 they start to become erratic and even start to rise again indicating that the engine is misfiring. 

Any suggestions on things that I should have looked at or test? My dual mag was overhauled 75 hours ago but I have a strong hunch it's the mag causing issues...

 

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Lean mixtures don't generally cause misfire if the engine conforms to specifications. There is always cycle-to-cycle combustion variation in each cylinder. Leaner mixtures have greater variation. This is the cause of the normal "roughness" one feels when LOP. As you go leaner, you eventually reach a point where cylinders just stop producing power.

A true misfire would be where a cylinder cuts in and out and I think this more likely an ignition issue. Leaner mixtures are more difficult to ignite and any weakness in the spark will show up intermittently.

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1 hour ago, KSMooniac said:

Who overhauled the mag? How about the harness? How old are the plugs, and who made them?

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Tempest plugs that are 75 hours old. Aero Accessories out of Van Nuys. Harness is also 75 hours. 

Edited by Evan
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The photo below is what I'm experiencing. As I approach 20 LOP the engine starts to run rough, then at about 30 LOP the #3 cylinders starts to rise in EGT. The others follow the deeper I go. My engine won't get to 50 LOP without the engine running really rough. 

image.png.7538d5a805c0b8918dbc18b6574f0115.pngHere is an interesting thread that Paul chimed in on. I'm pretty confident this is the exact problem I'm having. 

 

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1 hour ago, GeeBee said:

After you are LOP, what happens when you run single mag for a minute?

If I do my LOP mag check before engine roughness it looks like the below: 

image.png.7ef21e6c38f6b0710a4adbef1df75fbb.png

If I lean through engine roughness to get as close to 50 LOP, when I switch to one mag it pops and bangs. 

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54 minutes ago, Evan said:

The photo below is what I'm experiencing. As I approach 20 LOP the engine starts to run rough, then at about 30 LOP the #3 cylinders starts to rise in EGT. The others follow the deeper I go. My engine won't get to 50 LOP without the engine running really rough. 

image.png.7538d5a805c0b8918dbc18b6574f0115.pngHere is an interesting thread that Paul chimed in on. I'm pretty confident this is the exact problem I'm having. 

 

How fast and from what side are you leaning?  EGTs typically do rise again as lean combustion event becomes so slow that it continues to burn through the exhaust stroke.  It is entirely possible to lean through peak without actually getting to peak. Without complete data it’s hard to diagnose anything. I can smoothly run 100lop at 2000’’ MSL at 10,000 not so much but then, I would not have a reason to. The next time you fly, lean to rough and enrichen to just smooth. From the lean side slowly enrichen to peak on the richest cylinder. Take your time and really focus on finding peak. I suspect the onset of “lean roughness” may be quite a bit leaner than you think it is.

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8 hours ago, Shadrach said:

How fast and from what side are you leaning?  EGTs typically do rise again as lean combustion event becomes so slow that it continues to burn through the exhaust stroke.  It is entirely possible to lean through peak without actually getting to peak. Without complete data it’s hard to diagnose anything. I can smoothly run 100lop at 2000’’ MSL at 10,000 not so much but then, I would not have a reason to. The next time you fly, lean to rough and enrichen to just smooth. From the lean side slowly enrichen to peak on the richest cylinder. Take your time and really focus on finding peak. I suspect the onset of “lean roughness” may be quite a bit leaner than you think it is.

I try to lean no faster than .1 GPH per second. I usually do it from the rich side of peak. 

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4 hours ago, Evan said:

I try to lean no faster than .1 GPH per second. I usually do it from the rich side of peak. 

The first graphic suggests that you're getting a second EGT rise on all cylinders on the lean side of peak. From that we can say that all cylinders are sufficiently lean to produce a combustion event so slow that it is still burning when it enters the exhaust.  So, we can say with some certainty that you are able to lean all cylinders simultaneously to a point that is so lean as to barely support combustion. So, the question is why is it happening so close to peak?  It certainly could be ignition timing. If effective ignition timing is retarded well beyond 20 °, it could cause the symptoms you're experiencing.  However, I would think that you would notice other symptoms as well. Like reduced take off performance, lower CHTs and a decreased rate of climb.  I have never thought about how many GPH/sec I lean. The goal is to ascertain the actual peak EGT. I do it from the lean side because it ensures all cylinders are at a kind mixture setting while I'm leaning.  It's good to find actual peak occasionally but need not be done every flight.  

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1 hour ago, Shadrach said:

The first graphic suggests that you're getting a second EGT rise on all cylinders on the lean side of peak. From that we can say that all cylinders are sufficiently lean to produce a combustion event so slow that it is still burning when it enters the exhaust.  So, we can say with some certainty that you are able to lean all cylinders simultaneously to a point that is so lean as to barely support combustion. So, the question is why is it happening so close to peak?  It certainly could be ignition timing. If effective ignition timing is retarded well beyond 20 °, it could cause the symptoms you're experiencing.  However, I would think that you would notice other symptoms as well. Like reduced take off performance, lower CHTs and a decreased rate of climb.  I have never thought about how many GPH/sec I lean. The goal is to ascertain the actual peak EGT. I do it from the lean side because it ensures all cylinders are at a kind mixture setting while I'm leaning.  It's good to find actual peak occasionally but need not be done every flight.  

That’s the million dollar question. Really the only thing to try at this point is swapping mag, condensers, and new p leads. Other than those, not sure what else would cause this. 

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Set your MP so that yoi are 65% power or less and slllooowwwly lean from rich to lean. When i had to get my #3 injector changed to a more rich injector at gami the A&P went up with me to verify it was the correct size and in the leaning process he literally stopped leaning just before peak to let the mixture and more importantly the EGT to “catch up” to the mixture that was now set. I saw the egt rise another 50 degrees in that few moments and stabilize then he very slowly went another 1/4 turn and stopped and one of the first egt’s topped over peak to the inverted top down (jpi) then he turned another 1/8 and all the rest topped over.  I had a .2 spread but it would have been a bigger spread and it would not have been as high a peak egt temp if i had gone as fast as .1 gph per second like so many articles state to do the peak check. I will not go this slow above 65% as it takes too long in the red box area if i did. Try going slower and see if you are indeed way deeper lop than you originally thought. 

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13 minutes ago, Will.iam said:

Set your MP so that yoi are 65% power or less and slllooowwwly lean from rich to lean. When i had to get my #3 injector changed to a more rich injector at gami the A&P went up with me to verify it was the correct size and in the leaning process he literally stopped leaning just before peak to let the mixture and more importantly the EGT to “catch up” to the mixture that was now set. I saw the egt rise another 50 degrees in that few moments and stabilize then he very slowly went another 1/4 turn and stopped and one of the first egt’s topped over peak to the inverted top down (jpi) then he turned another 1/8 and all the rest topped over.  I had a .2 spread but it would have been a bigger spread and it would not have been as high a peak egt temp if i had gone as fast as .1 gph per second like so many articles state to do the peak check. I will not go this slow above 65% as it takes too long in the red box area if i did. Try going slower and see if you are indeed way deeper lop than you originally thought. 

A four cylinder Lycoming leaned to peak EGT from the lean side isn’t really going to heat up at any MP setting. Peak EGT provides nearly as much detonation margin as full rich. 
 

I think a lot of pilots are running leaner or richer than they think they are due to being nervous about leaning to peak.   Rushing the process of determining peak will lead to a false number.

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Just for kicks do a mag check at cruise leaned out, if one mag is the issue and I think that likely a mag check should show which one

.1 GPH per sec is way too fast, that’s 1GPH in 10 sec. or once again I made a stupid math error.

Here’s the problem it’s not a set leaning rate, as you approach peak, slow WAY down, probes have mass of course it takes a few sec for one to heat up and stabilize.

I lean a little and pause when close, lean a little and pause. Pretty quick you will notice that it seems to always peak at about the same place so leaning becomes much easier because within a very close margin you already know the fuel flow and temp it’s going to hit peak.

If this is an N/A  Lycoming don’t worry about being too hot, you can run all day at ANY mixture below 75%, to include peak, in my opinion if above 75% you don’t lean.

So you have two points, one where you run rich (above 75%) and the second, below 75% where you can’t hurt the motor with mixture.

Personally I’m risk adverse, Lycoming is on record for printing that for max engine longevity cruise at 65% or lower, so I do, and I don’t lean aggressively until cruise, if I’m in a long climb I’ll lean to tgt EGT, which just means lean to maintain takeoff EGT.

Long way of saying I don’t lean aggressively until 65% or lower

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On 11/22/2022 at 9:50 AM, Evan said:

That’s the million dollar question. Really the only thing to try at this point is swapping mag, condensers, and new p leads. Other than those, not sure what else would cause this. 

I had the identical problem early on with a newly Lycoming-rebuilt motor and new Slick mags (well, 125 hours old when I got it).  Same EGT rise just 10-20 dF LOP, and it would start feeling rough.  It ran fine anything richer than that, so I left it alone until I had hard starting.  I swapped out the left plugs with fine-wires with little change.  Luckily, when I mentioned it to my mechanic, he said he had a mag bench test rig and could easily check them.  I had him IRAN the left mag, and he noted the e-gap had drifted significantly from wear, even after just 150 hours.  When he retimed it the hard starts went away and the LOP improved.  When I had him IRAN the right mag around 400 hours, LOP improved further.

Now I can get down to around 50-60 dF LOP at 9000' MSL before I get the same effect (I don't typically run it that lean, though).  At low altitudes, I might get it down to only 20-30 dF LOP before getting EGT rise and roughness at higher power settings, so there are other variables, too.

The theory that it is a very slow normal combustion event caused by lean mixture would be disproven if fixing the e-gap timing corrects it at the same LOP mixture.  It does not make sense to me that the normal combustion event should suddenly become so slow just LOP with what amounts to a tiny drop in fuel:air ratio unless it's an abnormal combustion event, like one spark plug intermittently misfiring or two very weak sparks

Edited by jaylw314
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2 hours ago, Nippernaper said:

Not sure it’s been mentioned, but running LOP at high RPM can lead to roughness. Are you running 2400 or below?


Sent from my iPad using Tapatalk

While it’s true that reduced piston speed will allow the engine to run further LOP, any conforming IO360 should be able  to run leaner than it needs to for cooling at any cruise RPM.

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really nice collection of info!
 

One thing I didn’t see mentioned here….

What altitude is the LOP issue arising at?

At 5k’… my IO550 could operate smoothly until about 90°F LOP before it got really quiet….

At 10k’… the same IO550 would go about 50°F LOP before getting quiet….

To have the engine go quiet instead of running rough requires a Gami spread near 0.0 to 0.1….

 

Soooo…. The ability to go deep LOP is nice for demonstrating the ignition system is working really well….

How deep LOP do you want to go?

and… Why?

 

Above 8k’ the MP is naturally well below 65%…

Operating near peak, a few degrees LOP to burn up all of the 100LL…

CHTs are the only concern… staying under 380°F is nice….


With a 0.2 Gami spread… and multiple cylinders going cold at 40°F LOP….

You are seeing the limit of running LOP…. With this system.

The other question being raised… how did you find peak?  This is important because if you actually missed judge peak you might be 10°off… know that peak is pretty stable…. You can approach from either side, go away and come back…. It doesn’t move… (change altitudes, it moves)

if peak moves, find the technique where it is stable and reproducible…

Please, Let me know what I missed?   :)

PP thoughts only, not a CFI…

 

Best regards,

-a-

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On 11/21/2022 at 9:18 PM, Evan said:

 

Here is an interesting thread that Paul chimed in on. I'm pretty confident this is the exact problem I'm having. 

 

Tough engine comparison to make though….

They are using a a TC’d Continental in that conversation… and six cylinders vs. four won’t make the situation any easier…

Where the M20J’s power output varies with altitude…. So does its ability to go deep LOP…

The M20K’s engine can run far LOP, into the flight levels, at a high power output…  

Go turbo!

:)

Back to the question about what altitude you are flying at…

Best regards,

-a-

 

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While it’s true that reduced piston speed will allow the engine to run further LOP, any conforming IO360 should be able  to run leaner than it needs to for cooling at any cruise RPM.

I'm not sure why that would be. Mike Bush addressed this issue and indicated that a cirrus was experiencing issues from too high an RPM combined with LOP. I think it was NA, but dont recall if they were running excessively LOP.


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1 hour ago, Nippernaper said:


I'm not sure why that would be. Mike Bush addressed this issue and indicated that a cirrus was experiencing issues from too high an RPM combined with LOP. I think it was NA, but dont recall if they were running excessively LOP.


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I'll explain.  Assuming a well conforming (balanced A/F ratio across the cylinders), injected engine...any mixture setting that is sufficiently lean to cause the engine to run rough at a given given RPM has been leaned to a setting outside the range that is ideal for power, BSFC and cooling.  Starting at about 75ROP, adding air (i.e. leaning) slows the combustion event, as the mixture approaches peak EGT the effect of adding air (i.e. leaning) becomes more pronounced, which can be seen in the form of power loss and a reduction in CHTs   If piston speed (i.e. RPM) must be reduced for the engine to run smoothly, the engine was likely set too lean for the previous RPM.  I'll add that just because it runs smooth at reduced RPM does not mean that it is optimally set.   

A conforming engine becomes rough on the lean side of peak when the combustion event is so slow (relatively speaking) that much of the thermal energy is being released beyond the ideal crank angle range  at which it can be converted to mechanical energy.  So the engine runs rough and the EGT's start to rise again because much of the slow burning combustion event is being pushed into exhaust.  It's sort of like pedaling a bike but timing your pedal power pulse on the pedals for the last 20 degrees of the pedal's down stroke. You could shift into a higher gear to better match the pedal speed to your power pulses or you could simply reset your pedal power pulse to the appropriate time. 

So, as I said, If you have to reduce RPM to attain smooth operation at your desired FF, it's likely that that FF produced a setting that was just too lean for the previous RPM. Note that a given FF will yield an different mixture settings at a different RPMs.

I would be interested in reading the Busch article you've referenced.  I enjoy reading his work and am in philosophical agreement with most of his ideas on powerplant management.

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[
I would be interested in reading the Busch article you've referenced.  I enjoy reading his work and am in philosophical agreement with most of his ideas on powerplant management.


It’s in his “Mike Busch on Airplane Ownership Vol 2” in the chapter on operating oversquare. I can try and cut and paste the section here, maybe?


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46 minutes ago, Nippernaper said:

[
I would be interested in reading the Busch article you've referenced.  I enjoy reading his work and am in philosophical agreement with most of his ideas on powerplant management.


It’s in his “Mike Busch on Airplane Ownership Vol 2” in the chapter on operating oversquare. I can try and cut and paste the section here, maybe?


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Any help would be appreciated as long as it doesn’t violate Mike’s copyrights.

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